1. Field of the Invention
The invention relates to a hybrid electric vehicle powertrain with an engine, a multiple-ratio power transmission mechanism and an electric motor.
2. Background of the Invention
In a conventional automatic transmission for a road vehicle, the automatic transmission has multiple ratios, friction clutches and brakes to establish and disestablish multiple speed ratios in transmission gearing to maintain a desired engine speed and torque relationship and to achieve maximum engine performance throughout a range of vehicle speeds and varying torque requirements as the operating conditions for the vehicle change. The transmission clutches and brakes are engaged and released during a ratio shift. Engine torque is reduced at the beginning of a ratio shift event so that the friction clutches and brakes can be engaged and disengaged without excessive heat generation while minimizing undesirable inertia torque disturbances due to changes in angular velocity of the rotating mass of the transmission and powertrain elements involved in a ratio shift. Engine torque is reduced during a ratio shift by temporarily defueling the engine or, in the case of a spark ignition engine, by retarding ignition timing. As the shift progresses toward completion, the engine torque is restored.
If the automatic transmission includes a hydrokinetic torque converter with an impeller and a turbine connected respectively to the engine and a torque input element of transmission gearing, a torque converter bypass clutch may be used to bypass the hydrokinetic torque flow path by connecting the impeller to the turbine. If a shift occurs while the torque converter bypass clutch is engaged, the bypass clutch may be opened or partially opened to achieve an appropriate clutch slip during a transmission ratio shift interval. This is another well known technique for reducing inertia torque disturbances and improving a so-called quality of the ratio shift in the vehicle powertrain. A typical powertrain with a torque converter and a bypass clutch of this type may be seen by referring, for example, to U.S. Pat. No. 6,253,140.
An example of a hybrid electric vehicle powertrain that includes a traction motor and a transmission with multiple-ratio gearing is shown in U.S. Pat. No. 6,585,066. The powertrain of the '066 patent does not include a hydrokinetic torque converter, but it includes a slipping wet clutch between the rotor of an electric traction motor and the crankshaft of an internal combustion engine. The rotor is connected directly to torque input elements of a multiple-ratio transmission.
During a ratio shift of the powertrain of the '066 patent, the clutch may be slipped during a ratio change thereby attenuating inertia torque fluctuations in the powertrain resulting from changing angular velocity of the elements of a transmission that are involved in the ratio shift. This technique, as well as the technique described above, will provide better subjective shift quality, sometimes referred to as shift feel, which is detectable by the vehicle operator.
A powertrain such as that disclosed in the '066 patent relies upon the rotary kinetic energy of the rotor of the electric motor. Typically, the powertrain would have a single motor to complement engine power.